The present invention relates generally to an electrophotographic image forming apparatus, such as a copying machine, a facsimile machine, or a printer, and, more particularly, to an imaging forming apparatus and a method for controlling a developing bias voltage, a charging voltage, and the difference between the voltages.
Electrophotographic developing systems are generally employed in image-forming apparatuses such as photocopiers, laser beam printers (LBPs), light-emitting diode (LED) printers, and plain paper facsimile machines. The electrophotographic developing system operates to develop electrostatic latent images formed on a photosensitive medium into visible images using developers (such as toner) and transfers the visible images onto a printing medium such as paper. Such developing systems are mainly classified into a one-component developing system using a toner only, and a two-component developer, using a mixture of a carrier and a toner.
Such electrostatic image forming apparatus generally includes an image carrier implemented as a photoconductive drum or a photoconductive belt. A latent image is formed on the image carrier in accordance with image data. A developing device develops the latent image with a toner to thereby produce a corresponding toner image.
When an electrostatic latent image on a photosensitive medium is developed using negative charged toner, a developing bias voltage applied to a developing roller determines an amount of toner to be supplied to the photosensitive medium. For example, as shown in FIG. 1, the photosensitive medium 22 may be charged to a voltage of −750V. After the photosensitive medium 22 is exposed, the image area 24 of the photosensitive medium 22 may retain a voltage, Vexp, of −60 V. A developing bias voltage applied to a developing roller 20 is generally set to a voltage between charge on the photosensitive medium and image area, such as −450V, (i.e., between −750V and −60V).
The image area 24 may attract toner 26 from the developer roller 20, via; for example, force F1, such that developed toner 28 goes to the image area 24. Force F1 may result from a development potential between the voltage on the developer roller 20 and the voltage on the image area 24. For successful development, force F1 should be great enough to cause toner 26 to traverse a gap G between a developing roller 20 and the photosensitive medium 22. When development is attempted under these conditions, development of the toner 26 is driven by electric fields induced by the voltage difference between the developer roller 20 and the photosensitive medium 22.
When the voltage bias on the developer roller 20 is less than or equal to the voltage on the surface of the photosensitive medium 22, then a repulsive force, such as a surface potential on the surface of photosensitive medium 22, may act to impede or inhibit toner jumping from the developer roller 20 to a non-image area 30 of the photosensitive medium 22.
Additionally, when development is attempted under these conditions, electric fields may be controlled by precision in the size of the development gap. Toner charge distribution may be controlled by electrostatic triboelectric processes. The development gap precision and the toner charge distribution contribute to the enhancement of print quality. Without controlling either the development gap or the toner charge distribution, the printed image often suffers from poor dot formation and excessively thin lines within an image.
As can be seen, there is a need for an improved apparatus and methods for controlling developing bias voltage to solve the problem of degrading print quality, for example, a method that enhances image dot and line formation within an image.